Worldwide there are over 35 million individuals living with HIV. As many as 50% of these HIV-infected individuals will develop HIV associated neurocognitive disorders (HAND), despite combined antiretroviral therapy (cART). Yet, since the advent of cART the incidence of HIV associated dementia, the most severe form of HAND, has diminished and represents less than 5% of HAND in countries like the US where cART is available. Therefore, mild forms of HAND, such as Asymptomatic Neurocognitive Impairment and Mild Neurocognitive Disorder, now predominate. Eventually these mild forms of HAND lead to HIV associated dementia and its severe consequences. In addition, because HIV-infected individuals are living longer, they are susceptible to age related diseases like Alzheimer?s disease, which can exacerbate HAND. Consequently, adjunctive therapies [to cART] must be developed. Interferon-alpha (IFN?) is elevated in the cerebrospinal fluid of HAND patients and correlates with cognitive dysfunction. Studies, including both clinical and basic, have established that IFN? is neurotoxic causing cognitive dysfunction and neuronal dendritic abnormalities. Our investigations suggest that IFN? could be a target for adjunctive therapies for HAND. A model of HAND in SCID mice was developed and forms an important part of the translational component of this proposal. This model demonstrates behavioral similarities to HAND in humans. The model has been useful in studying pathogenesis and the development of novel treatments. Recent improvements using object recognition testing before and after treatment enable us to determine reversal of behavioral abnormalities by novel therapies. This aspect of the model is particularly important because it reflects mild cognitive impairment in humans with HAND, the most common forms, and thus models conditions occurring in human clinical trials. As a result, the HAND model represents a valuable tool for pre-clinical screening of novel adjunctive therapies. Also, IFN? is elevated in brains of HAND mice and blocking IFN? in HAND mice is an effective treatment that may prove effective in HAND patients. Nevertheless, neutralizing IFN? in humans may not be ultimately practical due to potential side effects. Therefore, rat neuronal cultures are used to study the mechanisms of IFN? neurotoxicity. By studying the mechanisms of IFN? neurotoxicity, novel approaches to treatment of HAND, and perhaps other cognitive disorders, may be developed. Studies have shown that IFN? neurotoxicity is mediated through both the IFN? receptor (IFNAR) and the NMDA receptor (NMDAR). Rat neurons exposed to IFN? exhibit decreases in dendritic length and branching. Recent work demonstrates decreased PSD-95 puncta along dendrites, suggesting more specific mechanisms of toxicity. Proteomics demonstrate decreases in Arf1, Cdc42, and ?-catenin, which are critical intracellular signaling proteins that are intimately involved in dendritic spine scaffolding. Arf1 decreases shown by proteomics have been verified through Western blotting. PSD-95 stabilizes NMDAR and AMPA receptors and thus PSD-95 decreases after IFN? potentially link it to Arf1, Cdc42 and possibly ?-catenin. After validation that Cdc42 and ?-catenin are decreased, we will overexpress all of them [individually] in neuronal cultures exposed to IFN? to determine if overexpression corrects PSD-95 decreases. We also hypothesize that because IFN? decreases PSD-95 on the spine, this leads to internalization of AMPA receptors both in vitro and in vivo. This ultimately results in disruption of neuronal networks, decreased plasticity, and long-term depression leading to poor cognition. We plan to investigate the relative contribution of the IFNAR and NMDAR to these effects, with the ultimate belief that both receptors contribute and that any future adjunctive treatments for HAND will need to address both receptors.